Photoinduced gratings are passive components of particular utility in optical fiber telecommunications systems. In essence, gratings are lengths of optical waveguide, such as optical fiber, in which periodic variations of the refractive index have been induced. When closely spaced, these periodic variations act as a Bragg grating and selectively reflect light having a wavelength of twice the spacing. When widely spaced, the variations act as a long period grating and can shift light in a first guided mode to a second guided mode or even to a non-guided mode. Such gratings can be used to filter, to define laser cavities, and to multiplex or demultiplex transmitted light.
Photoinduced gratings have been made in a variety of ways. An early approach was to form a reflecting surface on a short length of germanium glass optical fiber and to transmit a strong infrared laser beam down the fiber to set up a periodic interference pattern. Index perturbations form at the maximum intensities. See U.S. Pat. No. 4,474,427 issued to Kenneth O. Hill, et al which is incorporated herein by reference. A second approach is to direct two interfering beams of ultraviolet (UV) radiation through the cladding of an optical fiber to form an interference pattern along a germanium-doped glass core. See, for example, U.S. Pat. No. 4,725,110 issued to Glenn, et al which is incorporated herein by reference. A third technique is to subject periodic regions of a fiber core to UV radiation, as through an amplitude mask. See U.S. Pat. No. 5,104,209 issued to K. O. Hill, et al which is incorporated herein by reference. And yet another approach involves exposure of a fiber through a phase mask. See U.S. Pat. No. 5,327,515 issued to D. Z. Anderson, et al which is incorporated herein by reference.
U.S. Pat. No. 5,287,427 issued to P. J. Lemaire, et al. discloses that the index-changing effect of UV radiation can be enhanced by treating the glass with hydrogen or deuterium, and that in hydrogen or deuterium-treated glass (hereinafter generically referred to as hydrogenated glass) the index of refraction can be increased not only by UV radiation, but also by the application of heat.
U.S. Pat. No. 5,478,371 issued to P. J. Lemaire, et al. further discloses that the index of refraction of a glass region can be selectively increased by hydrogenating the glass and then simultaneously applying heat and UV radiation to the region.
A difficulty with the known techniques for making photo induced fiber gratings is that they typically require deep UV lasers which are highly expensive and difficult to control. Current grating writing takes place in the deep UV, at wavelengths between 190 and 260 nm. This is because fibers are most sensitive to UV in this spectral region because of absorption by germanium-oxygen deficiency centers (GODCs) at 240 nm.
Lasers in the deep UV region fall into three main classes: excimer lasers, excimer-pumped dye lasers and frequency-doubled argon ion lasers. All are expensive and difficult to use and align. In addition, the excimer lasers most commonly used to write gratings have poor beam quality and low coherence.
If gratings could be written using UV in the mid-UV region, at wavelengths in the range 300 nm to 400 nm and preferably 320-365 nm, then lasers such as argon ion lasers operating at 351 nm could be utilized. These lasers are simple, reliable, inexpensive, and have excellent beam quality and high coherence. He--Cd lasers at 325 nm could also be used although at lower power. Accordingly there is a need for an improved method of writing gratings which is operable using mid-UV lasers.